This invention relates to color photography. More particularly, this invention relates to color photography elements with induced spectral sensitization. This invention also relates to a novel process of forming a sensitizer useful in color photography. Furthermore, this invention relates to a novel color photography process.
The methods used in color photography are all based on the fact that the eye is essentially receptive to three basic colors: blue (between 400 and 500 millimicrons), green (between 500 and 600 millimicrons) and red (between 600 and 700 millimicrons). The subjective reproduction of a given color, i.e., a certain distribution of light energy, can be obtained by the reproduction of the same average value of energy density in these three bands.
Any color photographic process involves necessarily the two following steps:
(a) Analysis of the colors from the subject; and PA1 (b) Synthesis of the same colors. According to the mode of synthesis, one can distinguish between the additive and substractive processes. In addition systems the colors are formed by adding three fumdamental colors such as blue, green and red in the amount in the original subject. In subtractive systems the colors are formed by adding the complementary colors of the three fundamental colors blue, green and red found in the original subject. Looking at the image the eye then receives the complementary color of that existing on the print thereby restituting the original colors. The complementary colors of blue, green and red are respectively yellow, magenta and cyan. PA1 (a) with three filters and a single emulsion sensitive to a broad spectrum in the visible; and PA1 (b) with three emulsions sensitive to the three basic colors. In case (a) the filters can be incorporated into the emulsion in the form of a trichromic frame. PA1 (a) Developer+n Ag.sup.+ .fwdarw.n Ag+oxidized developer PA1 (b) Oxidized developer+Coupler.fwdarw.leuco dye PA1 (c) Leuco dye+Oxidizing agent.fwdarw.Dye PA1 (a) Phenol or naphtol derivatives leading to cyan dyes belonging to the indoaniline group PA1 (b) Methylene derivatives having a methylene activated function and leading to yellow and magenta dyes of the azomethines group. PA1 1. a red sensitized emulsion PA1 2. a green sensitized emulsion, and PA1 3. an unsensitized emulsion which is naturally sensitized to the blue or a blue sensitized emulsion, depending on the silver halide used. The red sensitized emulsion is more sensitive than the green which is itself more sensitive than the blue, in order to compensate for the light absorbed in the preceeding layers. PA1 UV to 450 nm for the blue, 530 nm for the green and 630 nm for the red. PA1 (a) The Fermi level E.sub.F is below the zero level or exactly below the electrostatic macropotential. In intrinsic semiconductors and insulators this level lies between the valence band and the conduction band. For n-type semiconductors, the position is below the conduction band (FIG. 3) and for the p-type semiconductors the Fermi level is raised by light (quasi Fermi level), and in p-type semiconductors, a decrease of the Fermi level under illumination is observed. The depth of the Fermi levels are obtained from the measurement of the thermoelectric work function .zeta. in vacuum. PA1 (c) The magnitude of the energy gap E between the valence band and the conduction band can be obtained from the onset of the absorption spectrum of the semiconductor or from the measurement of the optical threshold energy for photoconductivity. PA1 (d) The energy of the bottom of the conduction band is identical with the electron affinity .chi.. One can compute the electron affinity from the difference between the photoelectric work function .zeta. and the energy gap E: .chi.=.zeta.-E. PA1 (a) the appearance of a free electron in the excited level of the dye which may be able to travel through a close packed layer of molecules of dyes and the transition of the excited electron, the energy of which must be greater than that in the conductive state of the solid, to the semiconductor. PA1 (b) the recombination of the hole in the emptied ground level of the dye from the semiconductor.
The subtractive systems are practically the only ones still in use for photography. There are a number of color processes available to the industry today.
Color selection can take place in two different manners:
Color selection with three emulsions is the usual case. Each emulsion is sensitive to a basic color. A structure with three emulsions is represented in FIG. 1. The emulsions are generally separated by thin transparent layers with or without filter action.
Color synthesis can be obtained by selective generation of dyes, as it is usually the case, by modifying dyes already present in the emulsion, by selective discoloration or by selective masking.
The dyes formation involves a reaction between a developing agent and three complexing agents catalyzed by the presence of the photo-reduced silver in the emulsion. This reaction includes three different steps:
Developers are paraphenylenediamine derivatives having the general formula: ##STR1## R,R.sub.1 =alkyl, e.g., CH.sub.3, CH.sub.2 CH.sub.3 R.sub.2,R.sub.3,R.sub.5,R.sub.6 =alkyl and/or aryl which may be substituted or unsubstituted
Two types of couplers can be utilized:
The reaction mechanism is as follows: ##STR2## In basic pH the molecule of the developer is oxidized by the metal ion Ag.sup.+ thereby giving a diimine quinone ion stabilized by resonance: ##STR3## The following step is the reaction of the quinone diimine ion with the coupler ion. In basic pH, the coupler is strongly ionized: ##STR4## which allows for the coupling: ##STR5##